Because Vontee Davis ran a 4.49 at the combine, and and a 4.40 at his proday.
CHJ also ran a 4.48 at his pro day.
I watch butler run and Im fairly certain 9/10 times he would struggle to run under 4.5. He might come across the board with a 4.48-4.9 1 out of 10 tries. Im not saying he runs a 4.65 like he did at his pro day, but he doesnt run in the 4.4 range. On average he's in the 4.5 range, which isn't the end of the world.
Butler probably runs a really good 10 and 2o yard time, but he falls flat right after that. Yes Technique and everything play into effect when running the combine, and Im sure Butler wasn't a pro track star. However I can tell you confidently from watching him play, he does not have the Elite top end speed of a true #1, Man to Man, all on a island, elite Corner. Sure he can be great, but this is one portion of his game he (and the team) will always have to work around.